The invention relates to a throat and cone injector and a gas distribution grid containing same. More particularly, the invention relates to a throat and cone gas injector and to a gas distribution grid containing a plurality of these injectors useful for injecting a gas into a slurry which contains particulate solids, with reduced solids attrition and injector plugging.
There are many applications in which a gas must be injected up into a slurry which comprises particulate solids in a liquid. These include coal liquefaction, resid refining, other hydrogenation processes, slurry hydrocarbon and oxygenate synthesis, waste water treating, etc. The gas is typically injected up into the slurry by means of a horizontal gas distribution grid, which is also referred to as a plate or tray, located at the bottom of the slurry in a rector or other vessel, and over a gas space or cavity called a plenum. The tray contains a plurality of gas injectors extending therethrough for uniformly distributing the gas up into the slurry. These gas injectors can be vertical holes in the tray, bubble caps, and the like as is well known. Problems associated with these trays include solids attrition and plugging of the injectors if the gas pressure drops. While solids plugging of the gas injectors is never desirable, in some applications it is also important to avoid solids attrition. One specific example is a slurry hydrocarbon synthesis (HCS) process in which a synthesis gas (syngas) comprising a mixture of H2 and CO is bubbled up into and through a slurry in a reactor at reaction conditions effective to convert the syngas to hydrocarbons, at least a portion of which are liquid at the reaction conditions. In this process, the slurry comprises solid catalyst particles and gas bubbles in a hydrocarbon slurry liquid, with the slurry liquid comprising liquid HCS products. Reactors which contain a three phase slurry are sometimes referred to as slurry xe2x80x9cbubble columnsxe2x80x9d, as is disclosed in U.S. Pat. No. 5,348,982. The amount of syngas injected up into the slurry to provide the desired level of hydrocarbon synthesis is typically more than that needed to keep the catalyst particles dispersed in the liquid. Problems encountered with this (and other processes) include plugging of the gas injectors by the catalyst particles, catalyst particle attrition by the gas which produces fines and results in catalyst loss and plugging units downstream of the reactor, deactivation of catalyst settling on the flat spaces on the tray and catalyst falling down through the injectors into the plenum space below. It would therefore be an improvement to the art to use gas injecting means which reduce or eliminate any of these problems.
The invention relates to a gas injecting means useful for injecting gas into a slurry containing particulate solids, with reduced solids attrition and injector plugging. More particularly the invention relates to a throat and cone gas injector and to a gas distribution grid comprising a plurality of the gas injectors horizontally arrayed or distributed across, and vertically extending through, an otherwise gas and liquid impervious horizontal plate or tray, which will typically be located at the bottom of the slurry. In one embodiment, all or a portion of the gas injectors are formed as an integral part of the grid. The gas injector comprises a throat or elongated, hollow, first gas expansion zone open at both ends, with one end being a gas entrance having a bore through which gas is passed from outside the jet, past an orifice which is the exit of the bore, and into the throat, with the other, downstream end opening into an upwardly and outwardly extending second gas expansion zone, which may be cone-shaped. An internal shoulder at the junction of the throat and cone provides a flow diverting means, which directs slurry seeping down along the wall of the cone radially inward into the gas jet exiting the throat, to prevent slurry solids attrition and throat plugging. In a typical injector of the invention, the inner diameter of this shoulder is substantially the same as that of the throat and its outer diameter peripherally terminates at the inner wall of the bottom of the cone. It has been found that the absence of the shoulder, or flow diverting means in the gas injector, can result in significant and substantial disintegration of the particulate solids into fine particle sizes by attrition. These fines are carried up and out of the reactor in the overheads, and also pass through liquid filters into downstream units. This results in a continuous and substantial loss of catalyst and also forms sludge in downstream units which can clog the units and which must eventually be separated from the product. The pressure drop through the bore is determined by its diameter or cross sectional area, which is smaller than that of the throat. The aspect ratio and diameter of the throat are sized to (i) insure that the expanding gas jet flowing up through the throat contacts the inner throat wall before it exits the throat and enters the cone and, in combination with the orifice diameter. (ii) achieve the desired gas velocity in the cone where it contacts the slurry. This wall contacting in the throat is important to obtain a more uniform velocity profile of the gas stream entering the cone by allowing the velocity at the outer periphery of the gas jet to fully develop and to prevent flow perturbations and irregularities at the outer periphery of the jet which might otherwise result in solids weeping down into the throat, where they will be attrited by the higher velocity gas jet entering the throat through the narrow, smaller orifice. The throat has a length to diameter ratio (aspect ratio) of less than 10:1 and preferably less than 8:1 to prevent throat plugging by slurry solids. This aspect ratio must also be large enough for the outer periphery of the expanding gas jet to contact the inner wall of the throat before exiting it, and preferably make the contact a finite distance upstream of the throat exit, to provide a margin of safety (e.g., at least 10% of the throat length). It is preferred that the longitudinal axis of the bore, throat, shoulder and cone all be coincident. The angle of the interior cone wall is preferably greater than the angle of repose of the slurry solids, to prevent solids build-up in the cone. While in a preferred embodiment, the bore, throat, shoulder and cone all have a circular cross-section perpendicular to their longitudinal axis, other cross-sections may be employed if desired. A plurality of these injectors, vertically extending through and horizontally arrayed or arranged across a horizontal, and otherwise gas and liquid impermeable plate or tray at the bottom of the slurry, uniformly distribute gas up into the slurry, with a fairly uniform pressure drop and concomitant uniform gas injection velocity across and through each gas injector in the tray. This is determined by the desired gas throughput into the slurry, the number of injectors in the tray, the diameter of the injector bore, and the gas pressure upstream of the injector bore.
Close packing of the injectors across the grid will substantially reduce the amount of flat surface on the top of the grid. In a further embodiment, no part of the injector extends up past the top of the grid plate. These two features minimize the amount of dead space on the grid for slurry solids to settle on. In the case of a slurry hydrocarbon synthesis (HCS) process in which at least a portion of the solids are an HCS catalyst, the reduced solids settling results in less catalyst deactivation. The gas distribution grid is typically located above the synthesis gas (syngas) reservoir or plenum area at the bottom of the reactor. The syngas feed is passed into a plenum under the grid which serves as a pressure damper and which, in combination with the diameter of the orifice in the gas jets, the number of jets and gas pressure, insures that the amount of syngas entering each injector in the grid is the same.
Gas passing through the pressure reducing bore and into the throat or first expansion zone, exits the orifice as a radially outwardly expanding gas jet which flows through the throat and contacts the inner wall before it enters the cone. This contact is important, as it results in more energy being directed to the outer periphery of the jet exiting from the throat and into the cone, or second expansion zone. This provides sufficient gas velocity and mass (energy) at the periphery of the jet exiting the throat to prevent slurry from seeping down into the throat where the solids can be attrited into fines by the relatively high velocity gas jet entering the throat from the bore. It also reduces the difference in gas velocity between the outer periphery and the center of the jet, which further reduces particle attrition when the gas contacts the slurry. As the gas jet exits the throat, it doesn""t contact the inner cone wall at the throat and cone junction, because of the annular shoulder. In one embodiment, the inner periphery of the shoulder is adjacent the inner wall of the throat at its exit and the outer diameter peripherally terminates at the inner cone wall at the bottom of the cone. The shoulder acts as a flow diverter for slurry seeping down the wall of the cone by imparting a radially inward direction to the downflowing slurry. Slurry which flows down along the interior cone wall reaches the shoulder, which forces the downcoming slurry radially inward to where it meets the upflowing, expanding jet exiting the throat, which carries weeping the slurry back up into the slurry body above. As the expanding gas jet travels up through the cone, it contacts the slurry and breaks up into bubbles which have a net upward velocity greater than can be achieved with bubble caps and the like. In a further embodiment, the orifice opens into a cone or chamfer, or other means (depending on the shape of the orifice and throat) which prevents unrestricted expansion of the gas jet as it enters the throat, thereby preventing back swirls and eddies from forming which can be drawn into the expanding gas jet and disturb or cause perturbations, such as ripples, and other flow disturbances at the outer jet periphery as it travels up the throat and into the cone and also cause fluctuations in the jet itself. Such perturbations can permit slurry solids to weep down into the throat where they are attrited by the incoming higher velocity gas jet. The cone or chamfer thus stabilizes the expanding gas jet and has also been found to be beneficial in clearing the throat of solids due to reductions in the gas flow.